Conventionally, there is know a structure for mounting a battery in a vehicle, as in FIG. 6, in which left and right seat rails 101 are fixed to each of battery boxes 100 in advance, left and right seats 102 are mounted on these seat rails 101 in advance, and the seat rails 101 are fixed to a floor 103, whereby the battery boxes 100 and the seats 102 are mounted on a vehicle, thereby reducing the mounting time of the battery boxes 100 and the seats 102 (e.g., refer to Patent Literature 1). There is also known a structure in which batteries are disposed on both sides of a tunnel portion which extends in a front-to-rear direction of a vehicle in the center thereof, outward sides of the batteries are fixed to strength members on side portions of the vehicle, and the two batteries are connected together by a reinforcement member which straddles the tunnel portion, thereby increasing rigidity of a vehicle body (e.g., refer to Patent Literature 2). Further, there is also known a structure in which a case for accommodating a power control unit is positioned between a seat and a floor pan (e.g., refer to Patent Literature 3).